1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor film having a crystal structure and a method for fabricating a semiconductor device using the semiconductor film. Specifically, the present invention relates to a technique to increase crystallization and crystallinity by irradiating a semiconductor film with a laser beam. In addition, the present invention relates to a laser processing device which is used for the semiconductor film and the semiconductor device.
2. Description of the Related Art
A crystallization technique to form a polycrystalline silicon (polysilicon) by exposing an amorphous silicon film deposited on a glass substrate to a laser beam has been known. A pulsed excimer laser oscillator is a light source which is typically used for the crystallization technique. It is considered that silicon is instantaneously heated and melted due to the irradiation of a pulsed laser beam, and crystallization is occurred in the subsequent cooling process. In consequence, it is known that polycrystallization in which several crystal grains having different plane directions are collected is formed.
In crystallization of amorphous silicon using an excimer laser which is typically used, a silicon is selectively heated through irradiating with a pulsed laser beam with a pulse width of several ten nanosecond to several hundred nanosecond, thereby the silicon is crystallized without thermally damaging a glass substrate. The crystallization technique has attracted attention as a technique to form a semiconductor layer with a thin film transistor (TFT) which is used for a liquid crystal display device.
As shown in FIG. 5, a polysilicon film often used previously is formed by the following steps: forming a base insulating film 11 such as silicon nitride or silicon oxide over a glass substrate 10, irradiating an amorphous silicon film with a thickness of 50 nm formed over the base insulating film with an excimer laser beam at a laser pulsed frequency of 300 Hz (pulse width of 30 nsec), and then crystallizing the amorphous silicon film. Still, projections having a shape like mountain chain such as a projection 14 are formed onto the surface of the polysilicon film 13.
In the case of forming a TFT, a gate insulating film with a film thickness of approximately 100 nm is formed thereon. A decline of withstand pressure for an gate insulating film is caused due to the higher concentration of electric field at the projection area and the increased gate leak current. As the measure, there is a technique for preventing projections due to the segregation of impurities from generating onto the surface of the substrate by preventing the segregation of impurities with applying ultrasonic vibration to the substrate during crystallization by means of exposing the film to a laser beam. (Reference 1, Japanese patent Laid-Open 11-204433)
As previous experimental knowledge, it is known that a large grain size of crystallization is provided through crystallizing an amorphous silicon film in the atmosphere including oxygen by exposing the film to a laser beam.
A pulsed excimer laser is condensed into a linear shape through crossing optical lens to scan an amorphous silicon film. Thus, the whole surface of the amorphous silicon onto the glass substrate can be crystallized. However, the semiconductor film irradiated with the laser beam becomes high temperature and a state of solvent, and then, reacts with oxygen and nitride in the atmosphere. The resulted substance get into the film and the surface of the film, namely, exogenous impurities are mixed into the film and the surface of the film. Accordingly, the impurities causes a crystal defect or become a factor of damaging a quality of the crystallization due to the segregated impurities in the grain boundary.
Since these mixing of impurities occur unintentionally, the crystallized semiconductor film processed with a laser beam includes a local characteristic dispersion. Therefore, that becomes characteristic dispersion of a TFT formed by using the above semiconductor film, and a problem that characteristic dispersion among plural TFTs elements in the same substrate surface is caused.
In the above-described reference 1, it is described that segregation of impurities can be prevented and the occurrence of the projections onto the surface of a substrate due to the segregation of impurities can be prevented by applying ultrasonic vibration to the substrate. However, in the case of using a method to connect a ultrasonic vibration source with a stage on which a glass substrate is mounted, ultrasonic vibration source can not adhere to the stage because of a curve on the glass substrate, and ultrasonic vibration can not propagate efficiently. The tendency becomes increasingly prominent as the size of the glass substrate becomes bigger.